Isobutylene is an important basic organic chemical raw material. It has numerous derivatives. Its upstream and downstream industrial chains are complex. Its consumption structure is in diversified trends. From isobutylene, many products with high added value may be prepared, such as: butyl rubber, polyisobutylene, methyl tertiary-butyl ether, isoprene, polymethyl methacrylate and many other organic chemical raw materials and fine chemical products. As the market size of isobutylene downstream products keeps expanding, the imbalance between supply and demand will get more prominent. Particularly, under the background of increasing depletion of petroleum resources, the output of isobutylene has become a critical bottleneck holding back the development of downstream industry. Therefore, it is urgent to develop an isobutylene preparation route rather than a petroleum route.
Methane is a main component of natural gas, so methane conversion and utilization becomes an important research content of natural gas chemical technology. Particularly, in the recent years, under the general background of shale gas development and utilization, if isobutylene can be made from methane, it will be a new way to obtain isobutylene. However, methane has stable properties and is not easily activated, so it turns to be a bottleneck of chemical utilization of methane. Many domestic and foreign researchers have carried out the research of methane activation and conversion. The technology of halogen functionalization and then conversion of methane hopefully will become an important breakthrough to the technical problem of methane conversion.
From halomethane, many chemical products may be prepared. CN101041609A and CN101284232A disclose a method of converting methane into bromomethane under the action of oxygen and HBr/H2O and then taking further reaction of bromomethane to generate C3-C13 mixed high-carbon hydrocarbons. The selectivity of hydrocarbons of C5 or higher is 70%. HBr is used to bromize methane in the first reactor and released in the second reactor. After recovery, it is used in the first reaction again to realize cyclic use of HBr. Wang Ye et al (Jieli He, Ting Xu, Zhihui Wang, et. al. Angew. Chem. Int. Ed. 2012, 51, 2438-2442) discloses a modified molecular sieve catalyst of propylene from halomethane and preparation method thereof. By using a molecular sieve modified and treated with fluorinated compound to obtain an acidic catalyst containing an appropriate micropore structure, this catalyst may effectively catalyze halomethane and convert it into propylene. In the preparation and conversion of propylene from bromomethane, the single-pass bromomethane conversion rate of the prepared catalyst is 35-99% and the selectivity of propylene is 27-70%; in the preparation and conversion of propylene from chloromethane, the single-pass chloromethane conversion rate is 30-99% and the selectivity of propylene is 15-70%. Ivan M. Lorkovic et al (Ivan M. Lorkovic, Aysen Yilmaz, Gurkan A. Yilmaz, et al. Catalysis Today, 2004, 98, 317-322) also put forth a bromine circulation of using bromine to react with hydrocarbons in natural gas to generate bromo-hydrocarbons, then converting bromo-hydrocarbons into dimethyl ether, methanol and metal bromide on a metal oxide catalyst, and regenerating metal bromide by oxygen to obtain metal oxide and release simple substance bromine. At present, the target products of halomethane conversion in the existing literature are methanol, dimethyl ether, acetic acid, high-carbon hydrocarbon, ethylene and propylene. In the technologies in which low-carbon olefins with high added value are target products, the selectivity of a single product is not high. So far there is no report on highly selective synthesis of isobutylene from bromomethane.